A unique feature of the Model 22C is the use of a ratiometric AC resistance bridge to measure all types of sensors.

The AC excitation used with all resistor sensors (including Platinum RTDs) is a 1.625Hz bipolar square wave. This effectively eliminates DC offset errors including the thermal EMF induced offsets that often occur in cryogenic systems.

Positive Temperature Coefficient (PTC) resistor sensors including Platinum and Rhodium-Iron RTDs use the resistance bridge in it's constant-current, AC operating mode. Platinum RTD sensors use a built-in DIN standard calibration curve that has been extended to 14K for cryogenic use.

The Model 22C's resistance bridge is used in a constant-voltage AC mode to provide robust support for the Negative Temperature Coefficient (NTC) sensors commonly used by cryogenic applications. They include Ruthenium-oxide, Carbon-Glass, Cernox™, Carbon Ceramic, Germanium and several others. Since these sensors have a negative temperature coefficient, the use of a constant-voltage measurement method will reduce, rather than increase, power dissipation in the sensor as temperature decreases. By maintaining the lowest possible power level, sensor self-heating is minimized and useful temperature range is greatly increased.

An additional advantage to constant-voltage excitation is that NTC resistors lose sensitivity in the upper part of their range. By auto-ranging excitation current to maintain a constant voltage, sensitivity and noise immunity in that range is also improved.

The excitation source in the Model 22C is continuously variable so there are no steps in sensor self-heating.

Diode sensors are supported over their full temperature range by using the bridge in a DC, constant-current mode. This mode provides the required 10µA DC sensor excitation and 0 to 2.25 volt input range